1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Selection over an array of receivers
//!
//! This module contains the implementation machinery necessary for selecting
//! over a number of receivers. One large goal of this module is to provide an
//! efficient interface to selecting over any receiver of any type.
//!
//! This is achieved through an architecture of a "receiver set" in which
//! receivers are added to a set and then the entire set is waited on at once.
//! The set can be waited on multiple times to prevent re-adding each receiver
//! to the set.
//!
//! Usage of this module is currently encouraged to go through the use of the
//! `select!` macro. This macro allows naturally binding of variables to the
//! received values of receivers in a much more natural syntax then usage of the
//! `Select` structure directly.
//!
//! # Examples
//!
//! ```rust
//! #![feature(mpsc_select)]
//!
//! use std::sync::mpsc::channel;
//!
//! let (tx1, rx1) = channel();
//! let (tx2, rx2) = channel();
//!
//! tx1.send(1).unwrap();
//! tx2.send(2).unwrap();
//!
//! select! {
//!     val = rx1.recv() => {
//!         assert_eq!(val.unwrap(), 1);
//!     },
//!     val = rx2.recv() => {
//!         assert_eq!(val.unwrap(), 2);
//!     }
//! }
//! ```

#![allow(dead_code)]
#![unstable(feature = "mpsc_select",
            reason = "This implementation, while likely sufficient, is unsafe and \
                      likely to be error prone. At some point in the future this \
                      module will likely be replaced, and it is currently \
                      unknown how much API breakage that will cause. The ability \
                      to select over a number of channels will remain forever, \
                      but no guarantees beyond this are being made",
            issue = "27800")]


use fmt;

use core::cell::{Cell, UnsafeCell};
use core::marker;
use core::ptr;
use core::usize;

use sync::mpsc::{Receiver, RecvError};
use sync::mpsc::blocking::{self, SignalToken};

/// The "receiver set" of the select interface. This structure is used to manage
/// a set of receivers which are being selected over.
pub struct Select {
    inner: UnsafeCell<SelectInner>,
    next_id: Cell<usize>,
}

struct SelectInner {
    head: *mut Handle<'static, ()>,
    tail: *mut Handle<'static, ()>,
}

impl !marker::Send for Select {}

/// A handle to a receiver which is currently a member of a `Select` set of
/// receivers.  This handle is used to keep the receiver in the set as well as
/// interact with the underlying receiver.
pub struct Handle<'rx, T:Send+'rx> {
    /// The ID of this handle, used to compare against the return value of
    /// `Select::wait()`
    id: usize,
    selector: *mut SelectInner,
    next: *mut Handle<'static, ()>,
    prev: *mut Handle<'static, ()>,
    added: bool,
    packet: &'rx (dyn Packet+'rx),

    // due to our fun transmutes, we be sure to place this at the end. (nothing
    // previous relies on T)
    rx: &'rx Receiver<T>,
}

struct Packets { cur: *mut Handle<'static, ()> }

#[doc(hidden)]
#[derive(PartialEq, Eq)]
pub enum StartResult {
    Installed,
    Abort,
}

#[doc(hidden)]
pub trait Packet {
    fn can_recv(&self) -> bool;
    fn start_selection(&self, token: SignalToken) -> StartResult;
    fn abort_selection(&self) -> bool;
}

impl Select {
    /// Creates a new selection structure. This set is initially empty.
    ///
    /// Usage of this struct directly can sometimes be burdensome, and usage is much easier through
    /// the `select!` macro.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(mpsc_select)]
    ///
    /// use std::sync::mpsc::Select;
    ///
    /// let select = Select::new();
    /// ```
    pub fn new() -> Select {
        Select {
            inner: UnsafeCell::new(SelectInner {
                head: ptr::null_mut(),
                tail: ptr::null_mut(),
            }),
            next_id: Cell::new(1),
        }
    }

    /// Creates a new handle into this receiver set for a new receiver. Note
    /// that this does *not* add the receiver to the receiver set, for that you
    /// must call the `add` method on the handle itself.
    pub fn handle<'a, T: Send>(&'a self, rx: &'a Receiver<T>) -> Handle<'a, T> {
        let id = self.next_id.get();
        self.next_id.set(id + 1);
        Handle {
            id,
            selector: self.inner.get(),
            next: ptr::null_mut(),
            prev: ptr::null_mut(),
            added: false,
            rx,
            packet: rx,
        }
    }

    /// Waits for an event on this receiver set. The returned value is *not* an
    /// index, but rather an id. This id can be queried against any active
    /// `Handle` structures (each one has an `id` method). The handle with
    /// the matching `id` will have some sort of event available on it. The
    /// event could either be that data is available or the corresponding
    /// channel has been closed.
    pub fn wait(&self) -> usize {
        self.wait2(true)
    }

    /// Helper method for skipping the preflight checks during testing
    fn wait2(&self, do_preflight_checks: bool) -> usize {
        // Note that this is currently an inefficient implementation. We in
        // theory have knowledge about all receivers in the set ahead of time,
        // so this method shouldn't really have to iterate over all of them yet
        // again. The idea with this "receiver set" interface is to get the
        // interface right this time around, and later this implementation can
        // be optimized.
        //
        // This implementation can be summarized by:
        //
        //      fn select(receivers) {
        //          if any receiver ready { return ready index }
        //          deschedule {
        //              block on all receivers
        //          }
        //          unblock on all receivers
        //          return ready index
        //      }
        //
        // Most notably, the iterations over all of the receivers shouldn't be
        // necessary.
        unsafe {
            // Stage 1: preflight checks. Look for any packets ready to receive
            if do_preflight_checks {
                for handle in self.iter() {
                    if (*handle).packet.can_recv() {
                        return (*handle).id();
                    }
                }
            }

            // Stage 2: begin the blocking process
            //
            // Create a number of signal tokens, and install each one
            // sequentially until one fails. If one fails, then abort the
            // selection on the already-installed tokens.
            let (wait_token, signal_token) = blocking::tokens();
            for (i, handle) in self.iter().enumerate() {
                match (*handle).packet.start_selection(signal_token.clone()) {
                    StartResult::Installed => {}
                    StartResult::Abort => {
                        // Go back and abort the already-begun selections
                        for handle in self.iter().take(i) {
                            (*handle).packet.abort_selection();
                        }
                        return (*handle).id;
                    }
                }
            }

            // Stage 3: no messages available, actually block
            wait_token.wait();

            // Stage 4: there *must* be message available; find it.
            //
            // Abort the selection process on each receiver. If the abort
            // process returns `true`, then that means that the receiver is
            // ready to receive some data. Note that this also means that the
            // receiver may have yet to have fully read the `to_wake` field and
            // woken us up (although the wakeup is guaranteed to fail).
            //
            // This situation happens in the window of where a sender invokes
            // increment(), sees -1, and then decides to wake up the thread. After
            // all this is done, the sending thread will set `selecting` to
            // `false`. Until this is done, we cannot return. If we were to
            // return, then a sender could wake up a receiver which has gone
            // back to sleep after this call to `select`.
            //
            // Note that it is a "fairly small window" in which an increment()
            // views that it should wake a thread up until the `selecting` bit
            // is set to false. For now, the implementation currently just spins
            // in a yield loop. This is very distasteful, but this
            // implementation is already nowhere near what it should ideally be.
            // A rewrite should focus on avoiding a yield loop, and for now this
            // implementation is tying us over to a more efficient "don't
            // iterate over everything every time" implementation.
            let mut ready_id = usize::MAX;
            for handle in self.iter() {
                if (*handle).packet.abort_selection() {
                    ready_id = (*handle).id;
                }
            }

            // We must have found a ready receiver
            assert!(ready_id != usize::MAX);
            return ready_id;
        }
    }

    fn iter(&self) -> Packets { Packets { cur: unsafe { &*self.inner.get() }.head } }
}

impl<'rx, T: Send> Handle<'rx, T> {
    /// Retrieves the id of this handle.
    #[inline]
    pub fn id(&self) -> usize { self.id }

    /// Blocks to receive a value on the underlying receiver, returning `Some` on
    /// success or `None` if the channel disconnects. This function has the same
    /// semantics as `Receiver.recv`
    pub fn recv(&mut self) -> Result<T, RecvError> { self.rx.recv() }

    /// Adds this handle to the receiver set that the handle was created from. This
    /// method can be called multiple times, but it has no effect if `add` was
    /// called previously.
    ///
    /// This method is unsafe because it requires that the `Handle` is not moved
    /// while it is added to the `Select` set.
    pub unsafe fn add(&mut self) {
        if self.added { return }
        let selector = &mut *self.selector;
        let me = self as *mut Handle<'rx, T> as *mut Handle<'static, ()>;

        if selector.head.is_null() {
            selector.head = me;
            selector.tail = me;
        } else {
            (*me).prev = selector.tail;
            assert!((*me).next.is_null());
            (*selector.tail).next = me;
            selector.tail = me;
        }
        self.added = true;
    }

    /// Removes this handle from the `Select` set. This method is unsafe because
    /// it has no guarantee that the `Handle` was not moved since `add` was
    /// called.
    pub unsafe fn remove(&mut self) {
        if !self.added { return }

        let selector = &mut *self.selector;
        let me = self as *mut Handle<'rx, T> as *mut Handle<'static, ()>;

        if self.prev.is_null() {
            assert_eq!(selector.head, me);
            selector.head = self.next;
        } else {
            (*self.prev).next = self.next;
        }
        if self.next.is_null() {
            assert_eq!(selector.tail, me);
            selector.tail = self.prev;
        } else {
            (*self.next).prev = self.prev;
        }

        self.next = ptr::null_mut();
        self.prev = ptr::null_mut();

        self.added = false;
    }
}

impl Drop for Select {
    fn drop(&mut self) {
        unsafe {
            assert!((&*self.inner.get()).head.is_null());
            assert!((&*self.inner.get()).tail.is_null());
        }
    }
}

impl<'rx, T: Send> Drop for Handle<'rx, T> {
    fn drop(&mut self) {
        unsafe { self.remove() }
    }
}

impl Iterator for Packets {
    type Item = *mut Handle<'static, ()>;

    fn next(&mut self) -> Option<*mut Handle<'static, ()>> {
        if self.cur.is_null() {
            None
        } else {
            let ret = Some(self.cur);
            unsafe { self.cur = (*self.cur).next; }
            ret
        }
    }
}

impl fmt::Debug for Select {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Select").finish()
    }
}

impl<'rx, T:Send+'rx> fmt::Debug for Handle<'rx, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Handle").finish()
    }
}

#[allow(unused_imports)]
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests {
    use thread;
    use sync::mpsc::*;

    // Don't use the libstd version so we can pull in the right Select structure
    // (std::comm points at the wrong one)
    macro_rules! select {
        (
            $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
        ) => ({
            let sel = Select::new();
            $( let mut $rx = sel.handle(&$rx); )+
            unsafe {
                $( $rx.add(); )+
            }
            let ret = sel.wait();
            $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
            { unreachable!() }
        })
    }

    #[test]
    fn smoke() {
        let (tx1, rx1) = channel::<i32>();
        let (tx2, rx2) = channel::<i32>();
        tx1.send(1).unwrap();
        select! {
            foo = rx1.recv() => { assert_eq!(foo.unwrap(), 1); },
            _bar = rx2.recv() => { panic!() }
        }
        tx2.send(2).unwrap();
        select! {
            _foo = rx1.recv() => { panic!() },
            bar = rx2.recv() => { assert_eq!(bar.unwrap(), 2) }
        }
        drop(tx1);
        select! {
            foo = rx1.recv() => { assert!(foo.is_err()); },
            _bar = rx2.recv() => { panic!() }
        }
        drop(tx2);
        select! {
            bar = rx2.recv() => { assert!(bar.is_err()); }
        }
    }

    #[test]
    fn smoke2() {
        let (_tx1, rx1) = channel::<i32>();
        let (_tx2, rx2) = channel::<i32>();
        let (_tx3, rx3) = channel::<i32>();
        let (_tx4, rx4) = channel::<i32>();
        let (tx5, rx5) = channel::<i32>();
        tx5.send(4).unwrap();
        select! {
            _foo = rx1.recv() => { panic!("1") },
            _foo = rx2.recv() => { panic!("2") },
            _foo = rx3.recv() => { panic!("3") },
            _foo = rx4.recv() => { panic!("4") },
            foo = rx5.recv() => { assert_eq!(foo.unwrap(), 4); }
        }
    }

    #[test]
    fn closed() {
        let (_tx1, rx1) = channel::<i32>();
        let (tx2, rx2) = channel::<i32>();
        drop(tx2);

        select! {
            _a1 = rx1.recv() => { panic!() },
            a2 = rx2.recv() => { assert!(a2.is_err()); }
        }
    }

    #[test]
    fn unblocks() {
        let (tx1, rx1) = channel::<i32>();
        let (_tx2, rx2) = channel::<i32>();
        let (tx3, rx3) = channel::<i32>();

        let _t = thread::spawn(move|| {
            for _ in 0..20 { thread::yield_now(); }
            tx1.send(1).unwrap();
            rx3.recv().unwrap();
            for _ in 0..20 { thread::yield_now(); }
        });

        select! {
            a = rx1.recv() => { assert_eq!(a.unwrap(), 1); },
            _b = rx2.recv() => { panic!() }
        }
        tx3.send(1).unwrap();
        select! {
            a = rx1.recv() => { assert!(a.is_err()) },
            _b = rx2.recv() => { panic!() }
        }
    }

    #[test]
    fn both_ready() {
        let (tx1, rx1) = channel::<i32>();
        let (tx2, rx2) = channel::<i32>();
        let (tx3, rx3) = channel::<()>();

        let _t = thread::spawn(move|| {
            for _ in 0..20 { thread::yield_now(); }
            tx1.send(1).unwrap();
            tx2.send(2).unwrap();
            rx3.recv().unwrap();
        });

        select! {
            a = rx1.recv() => { assert_eq!(a.unwrap(), 1); },
            a = rx2.recv() => { assert_eq!(a.unwrap(), 2); }
        }
        select! {
            a = rx1.recv() => { assert_eq!(a.unwrap(), 1); },
            a = rx2.recv() => { assert_eq!(a.unwrap(), 2); }
        }
        assert_eq!(rx1.try_recv(), Err(TryRecvError::Empty));
        assert_eq!(rx2.try_recv(), Err(TryRecvError::Empty));
        tx3.send(()).unwrap();
    }

    #[test]
    fn stress() {
        const AMT: i32 = 10000;
        let (tx1, rx1) = channel::<i32>();
        let (tx2, rx2) = channel::<i32>();
        let (tx3, rx3) = channel::<()>();

        let _t = thread::spawn(move|| {
            for i in 0..AMT {
                if i % 2 == 0 {
                    tx1.send(i).unwrap();
                } else {
                    tx2.send(i).unwrap();
                }
                rx3.recv().unwrap();
            }
        });

        for i in 0..AMT {
            select! {
                i1 = rx1.recv() => { assert!(i % 2 == 0 && i == i1.unwrap()); },
                i2 = rx2.recv() => { assert!(i % 2 == 1 && i == i2.unwrap()); }
            }
            tx3.send(()).unwrap();
        }
    }

    #[allow(unused_must_use)]
    #[test]
    fn cloning() {
        let (tx1, rx1) = channel::<i32>();
        let (_tx2, rx2) = channel::<i32>();
        let (tx3, rx3) = channel::<()>();

        let _t = thread::spawn(move|| {
            rx3.recv().unwrap();
            tx1.clone();
            assert_eq!(rx3.try_recv(), Err(TryRecvError::Empty));
            tx1.send(2).unwrap();
            rx3.recv().unwrap();
        });

        tx3.send(()).unwrap();
        select! {
            _i1 = rx1.recv() => {},
            _i2 = rx2.recv() => panic!()
        }
        tx3.send(()).unwrap();
    }

    #[allow(unused_must_use)]
    #[test]
    fn cloning2() {
        let (tx1, rx1) = channel::<i32>();
        let (_tx2, rx2) = channel::<i32>();
        let (tx3, rx3) = channel::<()>();

        let _t = thread::spawn(move|| {
            rx3.recv().unwrap();
            tx1.clone();
            assert_eq!(rx3.try_recv(), Err(TryRecvError::Empty));
            tx1.send(2).unwrap();
            rx3.recv().unwrap();
        });

        tx3.send(()).unwrap();
        select! {
            _i1 = rx1.recv() => {},
            _i2 = rx2.recv() => panic!()
        }
        tx3.send(()).unwrap();
    }

    #[test]
    fn cloning3() {
        let (tx1, rx1) = channel::<()>();
        let (tx2, rx2) = channel::<()>();
        let (tx3, rx3) = channel::<()>();
        let _t = thread::spawn(move|| {
            let s = Select::new();
            let mut h1 = s.handle(&rx1);
            let mut h2 = s.handle(&rx2);
            unsafe { h2.add(); }
            unsafe { h1.add(); }
            assert_eq!(s.wait(), h2.id);
            tx3.send(()).unwrap();
        });

        for _ in 0..1000 { thread::yield_now(); }
        drop(tx1.clone());
        tx2.send(()).unwrap();
        rx3.recv().unwrap();
    }

    #[test]
    fn preflight1() {
        let (tx, rx) = channel();
        tx.send(()).unwrap();
        select! {
            _n = rx.recv() => {}
        }
    }

    #[test]
    fn preflight2() {
        let (tx, rx) = channel();
        tx.send(()).unwrap();
        tx.send(()).unwrap();
        select! {
            _n = rx.recv() => {}
        }
    }

    #[test]
    fn preflight3() {
        let (tx, rx) = channel();
        drop(tx.clone());
        tx.send(()).unwrap();
        select! {
            _n = rx.recv() => {}
        }
    }

    #[test]
    fn preflight4() {
        let (tx, rx) = channel();
        tx.send(()).unwrap();
        let s = Select::new();
        let mut h = s.handle(&rx);
        unsafe { h.add(); }
        assert_eq!(s.wait2(false), h.id);
    }

    #[test]
    fn preflight5() {
        let (tx, rx) = channel();
        tx.send(()).unwrap();
        tx.send(()).unwrap();
        let s = Select::new();
        let mut h = s.handle(&rx);
        unsafe { h.add(); }
        assert_eq!(s.wait2(false), h.id);
    }

    #[test]
    fn preflight6() {
        let (tx, rx) = channel();
        drop(tx.clone());
        tx.send(()).unwrap();
        let s = Select::new();
        let mut h = s.handle(&rx);
        unsafe { h.add(); }
        assert_eq!(s.wait2(false), h.id);
    }

    #[test]
    fn preflight7() {
        let (tx, rx) = channel::<()>();
        drop(tx);
        let s = Select::new();
        let mut h = s.handle(&rx);
        unsafe { h.add(); }
        assert_eq!(s.wait2(false), h.id);
    }

    #[test]
    fn preflight8() {
        let (tx, rx) = channel();
        tx.send(()).unwrap();
        drop(tx);
        rx.recv().unwrap();
        let s = Select::new();
        let mut h = s.handle(&rx);
        unsafe { h.add(); }
        assert_eq!(s.wait2(false), h.id);
    }

    #[test]
    fn preflight9() {
        let (tx, rx) = channel();
        drop(tx.clone());
        tx.send(()).unwrap();
        drop(tx);
        rx.recv().unwrap();
        let s = Select::new();
        let mut h = s.handle(&rx);
        unsafe { h.add(); }
        assert_eq!(s.wait2(false), h.id);
    }

    #[test]
    fn oneshot_data_waiting() {
        let (tx1, rx1) = channel();
        let (tx2, rx2) = channel();
        let _t = thread::spawn(move|| {
            select! {
                _n = rx1.recv() => {}
            }
            tx2.send(()).unwrap();
        });

        for _ in 0..100 { thread::yield_now() }
        tx1.send(()).unwrap();
        rx2.recv().unwrap();
    }

    #[test]
    fn stream_data_waiting() {
        let (tx1, rx1) = channel();
        let (tx2, rx2) = channel();
        tx1.send(()).unwrap();
        tx1.send(()).unwrap();
        rx1.recv().unwrap();
        rx1.recv().unwrap();
        let _t = thread::spawn(move|| {
            select! {
                _n = rx1.recv() => {}
            }
            tx2.send(()).unwrap();
        });

        for _ in 0..100 { thread::yield_now() }
        tx1.send(()).unwrap();
        rx2.recv().unwrap();
    }

    #[test]
    fn shared_data_waiting() {
        let (tx1, rx1) = channel();
        let (tx2, rx2) = channel();
        drop(tx1.clone());
        tx1.send(()).unwrap();
        rx1.recv().unwrap();
        let _t = thread::spawn(move|| {
            select! {
                _n = rx1.recv() => {}
            }
            tx2.send(()).unwrap();
        });

        for _ in 0..100 { thread::yield_now() }
        tx1.send(()).unwrap();
        rx2.recv().unwrap();
    }

    #[test]
    fn sync1() {
        let (tx, rx) = sync_channel::<i32>(1);
        tx.send(1).unwrap();
        select! {
            n = rx.recv() => { assert_eq!(n.unwrap(), 1); }
        }
    }

    #[test]
    fn sync2() {
        let (tx, rx) = sync_channel::<i32>(0);
        let _t = thread::spawn(move|| {
            for _ in 0..100 { thread::yield_now() }
            tx.send(1).unwrap();
        });
        select! {
            n = rx.recv() => { assert_eq!(n.unwrap(), 1); }
        }
    }

    #[test]
    fn sync3() {
        let (tx1, rx1) = sync_channel::<i32>(0);
        let (tx2, rx2): (Sender<i32>, Receiver<i32>) = channel();
        let _t = thread::spawn(move|| { tx1.send(1).unwrap(); });
        let _t = thread::spawn(move|| { tx2.send(2).unwrap(); });
        select! {
            n = rx1.recv() => {
                let n = n.unwrap();
                assert_eq!(n, 1);
                assert_eq!(rx2.recv().unwrap(), 2);
            },
            n = rx2.recv() => {
                let n = n.unwrap();
                assert_eq!(n, 2);
                assert_eq!(rx1.recv().unwrap(), 1);
            }
        }
    }
}